This invention is directed to pigment dispersions and in particular to pigment dispersions formulated with a graft copolymer having acid functionality.
Water dispersible polymers are well known in the art and have been used to form waterbased coating compositions, pigment dispersions, adhesives and the like. Graft copolymers containing carboxyl groups and the preparation of these polymers is shown in Japanese Laid Open Patent Application (Kokai) No. 1-182304 dated Jul. 20, 1989. This reference shows graft copolymers that have carboxyl groups based on acrylic and methacrylic acid in their side chains that have hydrophilic properties and teaches the use tertiary alcohol-based ester units of acrylic or methacrylic acid to form a macromonomer which is used to form a graft copolymer and then is hydrolyzed to form carboxylic acid groups on the polymer. The process taught by the reference is an inefficient process which does not form pure graft copolymer but results in a mixture of graft copolymer and low molecular weight components that are detrimental to pigment dispersions formed from the graft copolymer and finishes from such a composition.
Typical problems with water based coating compositions are as follows: the pigment dispersion used to form the composition often is not completely compatible with the film forming binder constituents of the composition; the polymeric dispersant does not uniformly disperse the pigments and the pigments flocculate or agglomerate; or the dispersant does not enter into the curing reaction and remains in the film as an unwanted component which later may leach out or cause deterioration of the resulting finish.
A polymeric dispersant is needed that will form an aqueous pigment dispersion that is stable and not flocculated or agglomerated, that is compatible with a variety of polymeric film forming binders conventionally used in water based coating compositions and that will cure with the film to form a finish of automotive quality that does not deteriorate on weathering because of adverse properties caused by the presence of the polymeric dispersant.